This invention relates to a seal type storage battery such as a nickel-cadmium storage battery, a nickel metal hydride type storage battery or the like, in particular, an improve seal type storage battery having a pressure detector for detecting inner pressure of the battery.
A seal type storage battery such as a nickel-cadmium seal type storage battery, a nickel metal hydride storage battery or the like produces oxygen gas at a positive electrode when close to charge up, and in some cases hydrogen gas is produced at a negative electrode, thereby increasing the inner pressure of the battery. For this reason, this kind of battery has a relief valve, which exhausts the gas out of the battery when the inner pressure exceeds a predetermined pressure, and the valve closes when the inner pressure is lower than the predetermined pressure.
In this case, a decrease of battery capacity occurs due to a decrease of electrolyte in accordance with the process of charge-discharge cycles. Therefore, various charging methods for preventing overcharge are employed. For example, a so-calledxe2x80x94delta-V method is employed to control the charge by detecting decreased voltage delta-V from a peak point in accordance with a phenomenon in which charged voltage shows the peak point at the end of charging. A method has also been employed that controls the charge by detecting surface temperature of the battery in accordance with the phenomenon that the surface temperature increases with the process of the charge.
However the charged voltage depends on charging current or ambient temperature, so that thexe2x80x94delta-V method described above has a problem in that dispersion of the detected delta-V value occurs at overcharge. On the other hand, the method that controls the charge by detecting the surface temperature of the battery, described above, can detect the difference of the temperature comparatively with accuracy when the charging current is small, but the temperature in short time charging increases quickly so that the method has the problem that an overcharge can occur due to a delay in detecting the increasing temperature.
Therefore, a method was proposed that controls the charge by detecting a rise of inner pressure of the battery due to the charge, for example in Japanese Laid-Open Patent Publication No. TOKU-KAI-HEI 5-153,734 (1993), Japanese Laid-Open Patent Publication No. TOKU-KAI-HEI 5-36,442 (1993) etc. However, in the method proposed in Japanese Laid-Open Patent Publication No. TOKU-KAI-HEI 5-153,734 (1993), a strain gauge is attached to the battery with adhesive to detect the inner pressure, and then deformation of the battery case is converted into variation of resistance so that charging is controlled by detecting the variation of resistance. In this case, it has a problem in that it cannot sufficiently prevent overcharge, since it cannot detect the inner pressure directly so that detection of the inner pressure is delayed, and sensitivity of the detection depends on the condition of the attachment of the strain gauge so that it cannot detect the inner pressure with certainty.
On the other hand, in the method proposed in Japanese Patent Application HEI 5-3644, 1993, it has a problem in that it cannot be used repeatedly when it cuts the charging current directly when charging with a huge amount of charging current, since a detection signal detected by a pressure detector controls a switching device, disposed in a charging circuit, to cut the charging current directly so that the switching device is susceptible to being broken. In addition, it is difficult to ensure that the pressure detector disposed in the trough will operate stably for a long time, so that it makes it difficult to detect its inner pressure with accuracy to protect the battery effectively. The detector is disposed under very extreme circumstances such as large variations of pressure or temperature, so that the detector is susceptible to aging. Further, the detector is exposed to electrolyte, which causes leakage.
Therefore, in order to solve the problems mentioned above, the object of the present invention is to provide a seal type storage battery that can detect the inner pressure of the battery caused by direct charging to prevent the occurrence of an overcharge.
To achieve the objects mentioned above, the seal type storage battery of this invention has a pressure-sensitive conductive rubber whose resistance changes continuously depending on a rise of inner pressure in the battery case. The conductive rubber is disposed in the battery case, and a lead connecting with the pressure-sensitive conductive rubber is extended to outside of the battery case. The lead which is extended to outside of the battery case, outputs a pressure detecting signal.
Since the seal type storage battery has the pressure-sensitive conductive rubber whose resistance is changed continuously depending on the rise of inner pressure of a battery case so that the detection does not delay because the inner pressure can be immediately output as the detecting signal. In addition, the pressure-sensitive conductive rubber detecting the pressure has superior resistance for transmutation of pressure and temperature so that the structure can prevent its aging. Further, as shown in FIG. 4, the pressure-sensitive conductive rubber has a characteristic that its resistivity changes in accordance with load (pressure) so that it can detect a rise of the inner pressure with accuracy. Especially, the resistitvity is extremely sensitive in the early stage at which load operates, so that the increasing inner pressure can be detected more precisely. Further, the pressure-sensitive conductive rubber shows a superior water-proof quality such that it can effectively prevent the degradation caused by the electrolyte. Accordingly, the seal type storage battery according to the present invention can detect the inner pressure of the battery with accuracy and immediacy using the pressure-sensitive conductive rubber. Thus, overcharging can be prevented for this type of battery over a long period of time with certainty. Furthermore, the seal type storage battery has a lead connected with the pressure-sensitive conductive rubber. The lead is disposed in the battery case and extended to the outside, so that simply connecting the extended lead with the control circuit of the charging circuit can cut the charging current, thus facilitating the connecting work between the seal type storage battery and charging circuit.
In this case, the seal type storage battery according to the present invention has a sealing equipment or assembly which has a sealing sheet having an opened hole in the center. The sealing assembly seals an open hole of the outer case via an insulative gasket and an electrode cap fixed on the outer surface of the sealing sheet. An alkali-resisting rubber sheet is disposed between the sealing sheet and the electrode cap, wherein the pressure-sensitive conductive rubber is disposed at the position of the opened hole of the sealing sheet and is sandwiched between the alkali-resisting rubber sheet and the electrode cap, so that the pressure due to gas produced in the battery can be transmitted to the pressure-sensitive conductive rubber via the alkali-resisting rubber. Therefore the gas pressure produced in the battery can be instantly transmitted to the pressure-sensitive conductive rubber immediately, and then output to the outside as an electrical signal via the lead connected to the pressure-sensitive conductive rubber.
Thereby, the seal type storage battery connected with the charging circuit can prevent overcharge, since the lead connected to the pressure-sensitive conductive rubber connects with the control circuit of the charging circuit so that the charging current can be cut immediately when the inner pressure of the battery exceeds a predetermined pressure value. In addition, even the sealing sheet has an open hole in the center of it, and the alkali-resisting rubber seals the open hole so that the internal battery is maintained in an airtight condition, and therefore the pressure-sensitive conductive rubber is not exposed to the alkali-condition of the internal battery.
In addition, the seal type storage battery may have a pressure-regulating valve assembly disposed between the sealing sheet, which is a component of the sealing equipment, and the electrode cap. The pressure-regulating valve equipment comprises a valve sealing the open hole portion of the sealing sheet and an elastic body biasing the valve to the open hole. In the valve, the pressure-sensitive conductive rubber is disposed in a position facing the open hole portion of the sheet via an alkali-resisting rubber. The seal type storage battery can cut the charge current immediately by detecting a rise of the inner pressure of the battery with the pressure-sensitive conductive rubber disposed inside of the valve. Further, in the seal type storage battery, the pressure-regulating valve can exhaust the gas in the battery case to the outside by opening the valve instantly, even if the charge current cannot be cut immediately in response to an increase of the inner pressure because of an error at the pressure-sensitive conductive rubber.
Furthermore, the seal type storage battery may seal the open portion of the outer case with the sealing sheet via a insulative gasket.